1. Field of the Invention
The present invention relates to magnesium alloys which has superior corrosion resistance and is superior in both heat resistance and casting properties, and magnesium alloy members produced using such magnesium alloys by various high-pressure casting methods such as metal injection molding, die-casting, and squeeze casting.
2. Related Art
Magnesium alloys are light in weight and superior not only in strength at room temperature but also in strength at high temperature. Thus, magnesium alloys are expected to apply to various applications. For example, heat-resistant members superior in corrosion resistance, such as transmission cases or oil pans, have been expected to put into practical use in the field of automobile. Such heat-resistant members can be formed from magnesium alloys so as to make a automobile body light in weight. As a result, the improvement of fuel consumption can be expected to contribute to suppression of global warming. In addition, in the field of household appliances, corrosion resistance and heat resistance are requirements for housings of liquid crystal projectors having light sources inside. Such housings can be formed from magnesium alloys. Thus, magnesium alloys can contribute to expansion of high-strength portable appliances. In other fields, magnesium alloys are expected to apply to light-weight members having corrosion resistance and heat resistance as requirements, such as machine tools or leisure goods.
As magnesium alloys of this type, in the conventional art, there were Al—Si-based alloys called AS41 and AS21, and Al—Mm-based alloys called AE42. Further, various alloys were proposed as follows, though they are not yet put into practical use.
Incidentally, contents of the following alloys are represented as “mass percent” by unit.    (1) An Mg alloy containing 1% to 6% of Al, 0.5% to 4% of Ca, 0.5% to 1.5% of Si, 0.15% to 0.5% of Mn and 0.1% to 0.3% of Zn (Japanese Patent Publication No. 17890/1991).    (2) An Mg alloy containing 2% to 10% of Al, 1.4% to 10% of Ca, 2% or less of Si, 2% or less of Zn and 4% or less of rare earth elements, providing Ca/Al≧0.7 (Japanese Patent Laid-Open No. 25790/1994).    (3) An Mg alloy containing 5% to 10% of Al, 0.2% to 1.0% of Si, 0.05% to 0.5% of Ca, and Sr ≦0.1% (Japanese Patent Publication No. 104942/1997).    (4) An Mg alloy containing 2% to 10% of Al, 1.0% to 10% of Ca, at least one of Si, Mn, Zn, Zr being ≦2%, and rare earth elements ≦4% (Japanese Patent Laid-Open No. 271919/1997).    (5) An Mg alloy containing 2% to 6% of Al, 0.5% to 4% of Ca, providing Ca/Al ≦0.8, and Sr ≦0.15% (Japanese Patent Publication No. 272945/1997).
Next, description will be made about operations of additional elements in the respective conventional alloys (including the proposed conventional alloys).
Al forms a hard intermetallic compound (Mg17Al12) together with Mg. Thus, its strengthened dispersion enhances the yield strength and the tensile strength of the alloy. Ca forms a high-melting-point intermetallic compound together with Al or Mg so as to enhance the tensile strength and the creep resistance. Si forms a high-melting-point intermetallic compound (Mg2Si) together with Mg so as to enhance the tensile strength and the creep resistance. Zn improves the aging ability. Rare earth elements (chiefly mesh metal: Mm) form an intermetallic compound together with Al so as to improve the creep resistance and the corrosion resistance as well as the elongation at high temperature.
Description will be made about problems of the additional elements in the conventional alloys.
Al is an element for improving the strength. However, excessive addition of Al result in increase of Mg17Al12 which is a low-melting-point and brittle intermetallic compound. Thus, the tenacity is reduced while the creep resistance is reduced.
Ca or Si has an effect to improve the tensile strength and the creep resistance a elevated temperatures . However, excessive addition of Ca result in not only decrease of the tenacity but also increase of the crack sensitivity during casting. Further, as the content of Ca increase, the corrosion resistance deteriorates suddenly.
Si forms a compound together with Ca so easily that considerable compound is crystallized during melting. Thus, the yield ratio of molten metal is reduced.
Zn is also an element for improving the strength. However, Zn lowers the creep resistance and increases the crack sensitivity during casting.
The rare earth elements are effective in improving the creep properties. However, the rare earth elements increase the material cost. In addition, the rare earth elements are oxidized so easily as to stick to a die. Moreover, conventional alloys were generally so high in melting point that the melting temperature had to be increased. Thus, molten metal burned easily. In addition, the solidus temperature was also so high that the fluidity of molten metal deteriorates. Thus, a casting failure was easily produced. Therefore, parts made of such alloys have not come to function for practical use.
Of such alloys, an Mg—Al—Ca alloy expected as a low-cost heat-resistant alloy containing no rare earth elements had a significant defect that addition of 2 mass % or higher of Ca required for obtaining satisfactory creep properties results in marked deterioration of the corrosion resistance.